(1) Field of the Invention
This invention relates to the deposition of metallic coatings from plating solutions. More particularly, this invention relates to wiping the cathodic coating surface during electrolytic coating and particularly to the use of a substantially solid wiper blade during such coating.
(2) Prior Art
A number of coatings are deposited from so-called plating baths in which a coating solution is subjected to an imposed electrical potential. Such imposed electrical potential basically enhances an already naturally occurring tendency for any metal ions in solution to deposit, or plate out, upon any metal object or surface immersed within or partially within the solution. Such metal surfaces are, under favorable conditions, able to supply electrons to metallic ions dissolved in the solution, converting such ions to less soluble metallic atoms which are deposited upon the electron donor material. This natural deposition, or plating out, of the coating material from a natural solution is invariably rather slow or, in many cases, even more than counterbalanced by simultaneously proceeding resolution processes. However, such natural deposition or plating rate can be improved dramatically by application of an external electrical potential to a plating bath, in effect causing a current to flow through, or partially through, the bath. The individual electrons of such current derived from the cathode combine with metallic ions in the coating bath adjacent to the cathode and rapidly convert such dissolved metal ions to metal atoms which deposit or plate out as a coating on the cathode from which the electrons are derived. Such externally applied current also more quickly forms metallic ions at the anode when a soluble anode is used, which ions dissolve into the coating bath to take the place of other ions deposited or plated out as a metal layer upon the cathode or other adjacent materials. So-called "electrolytic coating" using electrolytic coating baths is very widely used, both on a small scale and very large scale, for production-type coating of various products in large and small scale facilities.
Since the coating of a cathodic workpiece is largely merely the acceleration of a naturally occurring process or phenomena, fairly small changes in technique and apparatus accentuating those conditions that favor deposition and de-emphasizing these conditions that disfavor deposition, may have rather large effects upon the final coating obtained. The history of improvements in the field, therefore, is largely one of progressive small improvements and adjustments to improve the conditions for deposition of various coating metals on a metallic substrate temporarily included as the cathode in a plating circuit.
It has been found, for example, by the present inventors and others that it is conducive to good coating results to remove the hydrogen bubbles which are produced in an electrolytic solution at the cathodic work surface. Such bubbles are formed by transfer of electrons not only to the positive ions of the coating metal in the solution, but also to positive hydrogen ions in the electrolytic solution. Such positive hydrogen ions are derived from dissociated water, some ionization of which is always present in an aqueous solution, but which ionization is increased by the polarization of the water in the electrical field of an active electrolytic coating bath. The hydrogen collects initially as a thin cathodic film on the surface of the work and then with continued evolution of hydrogen tends to coalesce into macroscopically visible bubbles of hydrogen. The initial cathodic film is believed to be a combination or mixture of both hydrogen ions and atomic hydrogen. The thin cathodic film of hydrogen collecting upon the work surface, which film initially is only one atom thick, interferes to some extent with good coating in that it may tend to hold the larger metallic coating ions away from the surface. However, the hydrogen atoms are small and the layer of hydrogen is initially discontinuous so that their initial interference with coating is not too serious. In other words, it is relatively easy for a larger metallic ion to work its way in among the hydrogen. However, as more hydrogen is, in effect, precipitated out of the electrolyte onto the surface of the workpiece, interference with coating deposition by the hydrogen becomes greater and greater. In addition, the hydrogen tends to become incorporated into the coating itself both by having coating metal laid down about it and by migration through interatomic or intramolecular spaces into the coating metal and even into the base metal. It is well known that such interstitial hydrogen may, by straining the metallic space lattice, harden the metal, which may be advantageous in some cases, but is usually disadvantageous in that it may cause cracking.
If nothing is done to remove the hydrogen from the surface coating during the coating process, coating will usually continue, even though seriously interfered with by the hydrogen present, because such hydrogen as it accumulates tends to coalesce into larger local accumulations resulting in small bubbles and then larger and larger bubbles until such bubbles have sufficient volume and buoyancy to overcome their initial attraction for or clinging to the substrate surface and float upwardly in the solution to the surface to be finally dissipated in the surrounding atmosphere or local environment. Consequently, the hindrance caused by the presence of hydrogen gas at the surface of a cathodic workpiece does not tend to progress to the limit where it would cut off electrolytic plating completely. However, hydrogen is still a very significant hindrance to rapid coating or plating and the larger bubbles clinging to the surface of a workpiece may even lead to macroscopic pits and other defects in an electrolytic coating.
A second significant problem which has been long recognized in electrolytic coating baths is depletion of the electrolytic solution as coating progresses. In many cases, the only result is that the coating rate slows down due to there being progressively less coating metal ions in the solution to plate out. This has been counteracted by pumping in fresh coating solution, throwing in chunks of soluble coating metal for solution to "beef up" the electrolyte and other expedients. The trend has been to closer and closer control of the electrolyte composition during coating. Sometimes this has been implemented by continuous testing or analysis of the electrolytic bath as coating progresses. In addition, the coating solution baths have been mixed by impellers or the like, force circulated and re-circulated as well as frequently tested to hold them to a desired composition.
It has also been recognized that the coating bath next to a workpiece being coated may become locally depleted of coating metal ions and that such depletion may compromise efficient coating. Some installations have adopted the expedient of forced circulation of electrolyte past the point of coating or through a restricted coating area to increase the efficiency of coating. If the forced circulation is rapid enough, such circulation in itself also tends to detach bubbles of hydrogen from the cathodic coating surface, in effect, "killing two birds with one stone". However, the use of forced circulation of this type by pumps, jets and the like is not only unwieldy and expensive, but is believed by some to possibly have detrimental effects upon the coating itself because of the generalized rapidity of movement between the coating solution and cathodic workpiece, which macroscopically, at least, may interfere with efficient plating out of the metallic ions upon such work surface. Among the processes which have made use of rapid forced circulation is the so-called gap coating process in which a small coating gap between a coating anode and a cathodic workpiece is created and electrolytic solution is forced rapidly through such gap or opening.
Depletion of the coating solution has recently been found by one of the present inventors to be particularly serious in chrome plating solutions in which insoluble electrodes are used, since it has been found that unless the chromium plating operation is maintained substantially continuous and at a fairly uniform rate that hard chrome is difficult to efficiently plate out of a brush-type coating operation, or, for that matter, in semi-brush type operations. Additional details concerning the desirability of maintaining a constant electrolytic coating composition in the production of hard chrome coatings by brush plating are set forth in U.S. application Ser. No. 07/915,455 filed Jul. 16, 1992.
While various efforts to remove hydrogen bubbles from the coating surface in an electrolytic coating bath at the point of deposition have been tried, none has provided the ultimate quality of coating and efficiency of the coating operation which has been desired. Likewise, the ultimate in practical prevention of localized depletion in a coating bath has also not been attained. There has been a need, therefore, for a means for removing hydrogen bubbles and cathodic film from a cathodic coating surface as well as preventing localized depletion of the coating bath of coating material. The present applicants have found that a very effective means for accomplishing both these purposes is by the use of a relatively thin wiping blade applied to the surface of the workpiece at spaced intervals with a light contact. Such wiping blade deviates the relatively stable surface layer of electrolyte along a moving cathodic surface mixing and replenishing the electrolyte next to the cathodic surface. It also at the same time wipes or sweeps away bubbles of hydrogen as well as encourages coalescence of small bubbles and films of hydrogen into large bubbles for subsequent wiping away. Some of the more pertinent prior art patents related to the above noted problems and their solution are as follows.
U.S. Pat. No. 442,428 issued Dec. 9, 1890 to F. E. Elmore, discloses burnishing of the surface of a product being electroplated by impinging a burnishing implement against the surface being coated during the time coating deposition is proceeding. A core, mold or mandrel is mounted for rotation within a plating tank and a traveler arranged to move back and forth along the mandrel or the like as it rotates. The burnishing surface may be formed from agate, blood stone, flint or glass, in each case having a highly polished surface. These substances are characterized by Elmore as being non-conducting substances capable of burnishing and not acted upon by the coating electrolyte.
U.S. Pat. No. 817,419 issued Apr. 10, 1906 to O. Dieffenbach, discloses the use of comminuted kieselguhr in an electrolytic bath to act upon the surface of a workpiece during electrodeposition of metallic coatings. Dieffenbach mentions a previous German patent which added solid or liquid bodies to an electrolytic bath liquor which were able by impinging against a cathode, to remove small bubbles of hydrogen adhering to the workpiece or cathode as well as smoothing the metallic deposit. According to Dieffenbach, a previous German patent disclosed the use of sand, pumice-stone, brick dust, wood flour, and chaff as impinging substances. Dieffenbach states that his kieselguhr has the advantage over these other substances of being "much harder and sharper edged so that it is capable of cutting up more readily" than the other substances, "the small bubbles of hydrogen that are deposited on the cathode". He also indicates that kieselguhr becomes strongly impregnated with the coating liquid so that its specific weight is "reduced".
U.S. Pat. No. 850,912 issued Apr. 23, 1907 to T. A. Edison, discloses that during the plating of iron, the formation of gas bubbles frequently results in the coating being pitted or even perforated. In order to avoid such pitting by the formation of gas bubbles, Edison introduces a quantity of crushed charcoal into the solution which, he states, "will rub over and scour the surface of the deposited metal to polish the same and wipe off any gas bubbles which may tend to accumulate thereupon".
U.S. Pat. No. 1,051,556 issued Jan. 28, 1913 to S. Consigliere, discloses the use of a number of small, non-conducting bodies having rounded edges within an electrolytic coating bath, which bodies roll and beat on the surface of the body or "mold" upon which the metallic layer is being deposited or has already been deposited while the electric current is turned on. Consigliere suggests the use of glass or porcelain balls, ordinary pebbles and the like. He calls these bodies "burnishing bodies".
U.S. Pat. No. 1,236,438 issued Aug. 14, 1917 to N. Huggins discloses an apparatus for densifying electro-deposited material in which a roller positioned above the surface of the coating bath impinges upon the surface of a round body being coated as such body rotates out of the bath and wherein the surface is electroplated as the body rotates again down into the bath. Huggins states that for various reasons still undiscovered, but with which most of those skilled in the art are familiar, the metal deposited by the electrolytic bath is frequently spongy and unevenly deposited. Huggins'rolling process is said to be effective in consolidating the spongy material as well as the various layers which are separately laid down as the ring or roll rotates in the bath.
U.S. Pat. No. 2,473,290 issued Jun. 14, 1949 to G. E. Millard discloses an electroplating apparatus for plating crankshafts and the like with chromium in which a curved anode partially surrounds the portion of the workpiece to be coated. The curved anode has orifices in its surfaces to allow the escape of bubbles formed during the coating process and also has extending through its surface, a support for a so-called positioning block or scraper block 54 which is provided to maintain a close spacing between the anode and cathodic workpiece. Millard states also that his spacing block removes gas bubbles from the cathode and also removes threads of chromium. He also states that the block, which has a significant width, dresses and polishes the cathode during plating. The aim of Millard, is clearly to burnish or compact the coating surface somewhat in the manner of the earlier Huggins patent. While Millard talks, therefore, about scraping off the gas bubbles and also removing "threads" of chromium by which it is understood that he means dendritic material, he is primarily interested in conducting a burnishing operation and spacing his cathode from his anode by his relatively wide spacer block.
U.S. Pat. No. 3,183,176 issued May 11, 1965 to B. A. Schwartz, Jr., discloses the electrolytic treatment or coating of a bore by use of a brush coating apparatus mounted on a drill press. The inside of the bore is acted upon by a series of centrifugally extended rotating vanes having dielectric outer covers. The speed of rotation is very great and considerably higher than usually used in brush plating. The electrolyte is distributed to the surface of the vanes through the perforated cover.
U.S. Pat. No. 3,619,383 issued Nov. 9, 1971 to S. Eisner, discloses an electrolytic coating composition in which the surface of the strip which is being passed through an electrolytic coating tank is contacted with a special "activation" means which scratches the surface of the strip to activate such surface by, it is postulated or believed by Eisner, removing the polarization layer and distorting the metallic deposit in a manner which results in an increase in the rate of electrodeposition. The activation of the surface is provided by passing in contact with the strip an open weave fabric or compressed non-woven substrate having abrasive particles on the surface which scrape and plow the surface just as the electrodeposition takes place. The fibrous nature of the activating means also tends to draw along electrolyte with it so that the surface of the cathodic workpiece is always exposed to a fresh electrolyte. It is said that the activation process "precludes dendritic growth".
U.S. Pat. No. 3,699,015 issued Oct. 17, 1972 to N. E. Wisdom, discloses an electrodeposition process including the use of small "dynamically hard particles having a vibratory motion". It is said such treatment considerably increases the throwing power of the electro-deposition process. Wisdom discloses the prior use of small glass spheres, sand and the like to beat the electrolytic material deposited upon the coating surface within a vibratory chamber and make it more dense and coherent, but indicates that his vibratory particles are superior. The vibration coats particles with the electrolytic solution and carries it apparently to the pieces being coated which are supported above the nominal solution level, but within the accumulation of particles.
U.S. Pat. No. 3,699,017 also issued Oct. 17, 1972, to S. Eisner, refers to both the prior art and the preceding Wisdom patent and discloses that he makes use of a deposit particle having a body or core portion formed of an electrically conductive material which is usually, but not necessarily, the same metal as intended to be deposited. Over such core is a protective outer covering or sheath of non-conductive material which is sufficiently hard to permit the particles to act as "dynamically hard". It is said that the particle cores, being electrically conductive, provide reasonably direct passage for the current flow and the particles themselves act as bipolar electrodes.
U.S. Pat. No. 3,734,838 issued May 22, 1973 to S. Eisner, is an improvement on his previous '383 patent in which small abrasive particles held on a fibrous or woven strand are used to remove or abrade away a "depleted ion layer". In the '383 patent, small particles of electrolyte are introduced to the plating zone in small discrete volumes. The process is said to be useful in depositing alloys as distinguished from single metal or ion coatings.
U.S. Pat. No. 3,749,652 issued July 31, 1973 to S. Eisner, discloses a further method of forming soft chromium deposits which are not as subject to cracking as hard chrome. Eisner uses in one embodiment at least, a mechanical activator disk formed of Dacron fibers and carrying a coating of 600 grit silicon carbide abrasive secured to the Dacron fibers by a polyurethane adhesive. The disk is rotated against the end of the rod during electrodeposition and is indicated to result in a superior non-cracking coating.
U.S. Pat. No. 3,751,346 issued Aug. 7, 1973 to M. P. Ellis et al., discloses an arrangement by which a combined plating and honing procedure may be followed. In the arrangement, a plurality of honing stones are arranged to be movable into contact with the surface of the workpiece during the actual plating operation resulting in better surface characteristics, superior, it is said, to what was obtained before.
U.S. Pat. No. 3,753,871 issued Aug. 23, 1973 to S. Eisner, provides a somewhat different embodiment of the Eisner vibrating activation particles to activate the surface. Hard outer layers are formed over softer inner layers in the new Eisner particles.
U.S. Pat. No. 3,769,181 issued Oct. 30, 1973 to J. L. Biora et al., discloses simultaneously machining and electroplating a workpiece by using a rotatable machining tool applied against the surface of the workpiece being electroplated at the same time as an electroplating solution under a high current density is applied to the machining zone. It is stated that high deposition speeds are possible because of the strong agitation provided by the apparatus in a very short distance or restricted area between the anode and the cathode which permits the use of ten times the current densities used in normal plating processes. The machining tool is called in some places a honing tool and it is stated that this can be used as the anode. Various increases in properties are alleged.
U.S. Pat. No. 3,772,164 issued Nov. 13, 1973 to M.P. Ellis et al., discloses the use of honing stones which hone the surface of a workpiece as an electrolytic coating is being applied.
U.S. Pat. No. 3,886,053 issued May 27, 1975 to J. M. Leland, discloses a method of electrolytic coating involving pulsing the current through an electrolyte containing a chromium plating solution while simultaneously performing a honing operation. The hydrogen derived from the coating process is allowed to escape intermittently during the reduced current period in order to avoid buildup of stress and provide a softer plated coating adjacent to the workpiece. It is disclosed by Leland that the honing of a chromium coating, for example, allows a high current density and faster deposition than the normal electrolytic tank process. The higher hardnesses, states Leland, of honed-forming processes have been attributed to the mechanical work introduced in the plating process by the honing operation. This, however, locks in residual tensile stress and adversely affects the junction between the metal base and the plating causing adhesion failures. Leland indicates that he has found that by providing an on-and-off current period by pulsing the plating current, a softer coating is provided. The mechanism as explained by Leland comprises essentially the deposition during the deposition period in chromium plating of chromium hydride (CrHx) on the base metal. During the subsequent non-deposition period, the CrHx, being thermally unstable, is afforded time to decompose and the hydrogen gas is allowed to escape before the commencement of the next deposition period.
U.S. Pat. No. 4,125,447 issued Nov. 14, 1978 to K. R. Bachert, discloses the use of a brush attached to a movable anode within a hollow member being electroplated. The brush comprises a plurality of bristles made from plastic or other insulated material which rub against the inside surface of the tube being electroplated as the anode vibrates. This, it is said, provides an agitation, scrubbing and/or washing action inside the tube which tends to remove any plating material that does not have good adhesion and results in a uniform plated surface on the tube.
U.S. Pat. No. 4,176,015 issued Nov. 27, 1979 to S. Angelini, discloses the brushing of the surface of a series of bars as they are passed in a straight line through an anode immersed within an electroplating bath. The brushing is provided by a brush comprising a blade having a layer of fiber or the like scraping material compressed between side plates. Such brush material is made of acid resistant material from which the glass fibers protrude only as much as necessary to touch the surface of the bars to be polished. It is said that the removal by the action of the brush of the cathodic film on the surface of the bars remarkably improves the plating process and the quality of the chromium layer on the bar surface. The cathodic film is formed, according to Angelini, of hydrogen ions which interfere with the plating current flow consequently hindering the electro-deposition of the chromium. As indicated, the brushing device removes such cathodic film.
U.S. Pat. No. 4,210,497 issued Jul. 1, 1980 to K. R. Loqvist et al., discloses the coating of hollow members including movement inside the cavity of such members of an electrolytic solution by means of a "conveyor" which consists of a resiliently and electrically insulating material such as perforated, net-like or fibrous strip which is wound helically around a reciprocating anode. The strip is fringed or slit on the edges facing towards the cavity wall to form fingers extending outwardly into contact with the cavity wall. It is said that the helical arrangement of the strip aids in conveying foam and gases formed during plating with high current density out of the cavity. It is also stated that in order to increase the rate at which the electrolyte, foam and gases are transported, the workpiece along with the anode and the fringe strip about it can be arranged vertically or at a suitable inclination calculated to aid the removal apparently of the gases. It is also stated that the gas conveying and electrolytic conveying material can consist of various types of perforated fibers or net-like bands other than the plastic strip mentioned and that the function of the resilient electrically insulated material is to act as a conveyor of electrolyte, foam and gases which can be supplemented by forming the anode as a screw conveyor. Furthermore, it is stated, several conveyors can be arranged in the cavity.
U.S. Pat. No. 4,595,464 issued Jun. 17, 1986 to J. E. Bacon et al., discloses the use of a so-called brush belt for continuously treating a workpiece. The brush belt is in the form of a continuous loop which passes over suitable rollers or pulleys and brings plating solution in the brush portion to the plating area. The brush is formed of a highly absorbent material which is chemically inert to the plating solution. It is stated that an open-cell urethane foam or other materials such as felt or neoprene is preferred. The absorbent material must be capable of allowing the solution to pass through one side to the other and be held by the material. It is said that the belt may be driven in a direction opposite to the workpiece at a speed that will most effectively break down the cathodic film buildup on the interface or contact point between the brush belt and the web workpiece. It is also stated that a squeegee apparatus may be placed at a location on the brush belt after it passes by the supply of plating solution to squeeze out plating solution remaining on the belt after the plating operation. Essentially, therefore, Bacon et al. provides an absorbent belt which passes in opposition to the material to be coated.
U.S. Pat. No. 4,853,099 issued Aug. 1, 1989 to G. W. Smith discloses a so-called gap coating apparatus and process in which a relatively small elongated gap is established through which coating solution is passed at a high rate. It is said that the ultra high flow rate allows very high current densities. It is stated the process is not well suited for chromium plating, because high current densities do not increase the plating out of chromium.
U.S. Pat. No. 4,931,150 issued Jun. 5, 1990 to G. W. Smith, discloses a so-called gap-type electroplating operation in which a selected area of workpieces is coated by forming an electrode closely about such so-called gap and passing electrolytic solution through the gap at a high rate. It is stated that the ultra-high volume flow assures the removal of gas bubbles, the maintenance of low temperature and high solution pressure contact with the anode surface and a workpiece surface. It is stated that gaps approaching two and one half inches can employ the invention, but the gap would preferably be smaller, but at least 0.05 inches in width. It is stated that a fresh plating solution having a controlled temperature and no staleness is available at all times in the gap for uniform plating and while in high pressure contact with the surface of the gap. In practice, the plating solution is forced in a vertically upward direction so that any gas generated by the electrolysis in the gap migrates upwardly in the same flow direction as the plating solution is being driven and, therefore, can readily escape. It is also stated that chromium is difficult to use in the invention because chromium deposits slowly regardless of current density so that the deposition is slow and the advantages of gap plating are not fully attained.
While other processes and apparatus have, therefore, been available to both to remove hydrogen bubbles from cathodic coating surface, sever and remove dendritic material in coating processes such as the electrolytic coating of chromium and prevent depletion of the electrolytic solution, all such prior processes have had drawbacks and none has been effective to accomplish all three or even two of the disclosed aims by themselves.